In today's communication networks many different kind of services are existent and employed by a steadily growing number of users. Typically, these services are offered by service providers with which users may register in order get access to requested services. In general, a user's registration with a service provider is an anonymous process in a certain sense, i.e. the registration is based on a user's digital identity, whereas the user's real identity remains unknown to the service provider.
For many applications in the digital world, such as for instance e-voting, e-cash, electronic coupons or trial browsing of content, it is a crucial task to impose a strict limit on the number of different digital identities with which a user may invoke or register with a service. For instance, in case of e-voting it is essential to reliably assure that each user can vote only once. In case of trial browsing of content providers typically want to make sure that a specific user is allowed to browse content such as movies or music freely only for a limited number of times, e.g. the service provider may allow the user to download two movies or ten songs. After the trial phase the service provider wants the user to pay for the service.
According to the state of art there are solutions for enabling a limitation of service access that are based on asymmetric cryptography. However, these methods known from prior art are rather disadvantageous as the employed asymmetric techniques require expensive computations and complex key management procedures resulting in a high computational overhead. Due to their computational complexity, these methods are not suitable for many applications.
Furthermore, imposing a limit on the number of different digital identities with which a user may invoke or register with a service typically requires the service provider to obtain certain information about a registered user. This links the user's digital identity employed for accessing the service provider at least to some extent to the user's real identity. Such link may be critical with respect to preserving a user's privacy.